Particle beam devices are used for examining samples (hereinafter also called objects) in order to obtain insights with regard to the properties and behaviour of the samples under specific conditions. One of those particle beam devices is an electron beam device, in particular a scanning electron microscope (also known as SEM).
In an SEM, an electron beam (hereinafter also called primary electron beam) is generated using a beam generator. The electrons of the primary electron beam are accelerated to a predeterminable energy and focused by a beam guiding system, in particular an objective lens, onto a sample to be analyzed (that is to say an object to be analyzed). A high-voltage source having a predeterminable acceleration voltage is used for acceleration purposes. Using a deflection unit, the primary electron beam is guided over a surface of the sample to be analyzed. In this case, the electrons of the primary electron beam interact with the material of the sample to be analyzed. In particular, interaction particles and/or interaction radiation arise(s) as a consequence of the interaction. By way of example, electrons are emitted by the sample to be analyzed (so-called secondary electrons) and electrons of the primary electron beam are backscattered at the sample to be analyzed (so-called backscattered electrons). The secondary electrons and backscattered electrons are detected and used for image generation. An image of the sample to be analyzed is thus obtained.
Furthermore, ion beam devices are known which comprise an ion beam column. Using an ion beam generator arranged in the ion beam column, ions are generated which are used for processing a sample (for example for removing a layer of the sample or for depositing material to the sample, wherein the material is provided by a gas injection system) or else for imaging.
Moreover, it is known from the prior art to use combination devices for processing and/or for analyzing a sample, wherein both electrons and ions can be guided onto a sample to be processed and/or to be analyzed. By way of example, it is known for an SEM to be additionally equipped with an ion beam column as mentioned above. In this case, the SEM serves, in particular, for observing the processing, but also for further analysis of the processed or non-processed sample. Electrons may also be used for depositing material. This is known as Electron Beam Induced Deposition (EBID).
There is an increasing demand for large images of large samples generated using a particle beam device. However, since a particle beam may normally be guided over a part of the surface of the sample to be analyzed only, it is known to generate sub images of the sample first, and subsequently to stitch the sub images together to a large image of the sample comprising all the sub images. The sub images are also known as tiles and are generated in such a way that they comprise a common region. For example, it is known to generate a first sub image and a second sub image in such a way that they have a common region. They are stitched together in the common region along their peripheries. The success of this stitching technique depends on how well the generated first sub image and the second sub image can be aligned along the common region. This alignment is also known as registration. The method is known as image stitching.
The known image stitching method may be automatically performed using software comprising image recognition and coordination algorithms. However, for such software to work, the common region of the first sub image and the second sub image should comprise distinct features to allow the image recognition and coordination algorithms to yield a sufficiently result. The distinct features are used as orientation marks (also known as orientation markings) for determining a unique relative position of the first sub image to the second sub image. However, the common region of the first sub image and the second sub image may not have such distinct features or only features which are not very useful for the known image stitching method. For example, a common region of the first sub image and of the second sub image may comprise at least one of the following:    (i) Essentially no or no usable features. Therefore, it may not be possible to find a unique alignment of the first sub image to the second sub image in the area of the common region since there are no or no usable distinct features as orientation marks in the common region. Indeed, no or no usable features may lead to several possible alignments of the first sub image to the second sub image as shown in FIGS. 1A and 1B. A common region CR of the first sub image SI1 and of the second sub image SI2 does not comprise any distinct feature. Therefore, for example, the first sub image SI1 may be aligned to the second sub image SI2 as shown in FIG. 1A or as shown in FIG. 1B. Accordingly, the known stitching method may not reveal a sufficient outcome. It may not be possible to determine a unique relative position of the first sub image SI1 to the second sub image SI2.    (ii) Features that are repetitive. Such features are often found on semiconductor circuit objects. FIGS. 2A and 2B show a first sub image SI1, a second sub image SI2 and a common region CR comprising a repetitive feature RF. In this case, the alignment of the first sub image SI1 to the second sub image S12 in a specific direction will generally be uncertain by n×P, where P is the repetition in the specific direction and n is a natural number. Therefore, for example, the first sub image SI1 may be aligned to the second sub image S12 as shown in FIG. 2A or as shown in FIG. 2B. Accordingly, the known stitching method may not reveal a sufficient outcome. It may not be possible to determine a unique relative position of the first sub image SI1 to the second sub image S12.    (iii) Features that extend from the first sub image to the second sub image along a common straight line. Those lines are often found on crystalline samples. FIGS. 3A and 3B show a first sub image SI1, a second sub image SI2 and a common region CR comprising a line L. In this case, the size of the common region CR is unclear. For example, the first sub image SI1 may be aligned to the second sub image S12 as shown in FIG. 3A, wherein the common region CR has a first size, or as shown in FIG. 3B, wherein the common region CR has a second size. Accordingly, the known stitching method may not reveal a sufficient outcome. It may not be possible to determine a unique relative position of the first sub image SI1 to the second sub image S12.
If there is no distinct feature which may be used as an orientation mark, the known image stitching method may comprise the step of generating distinct features on the object, wherein the generated distinct features may be used as orientation marks in the known image stitching method. However, the object as such is modified which may not be desired. Moreover, the generated orientation marks may not be removable from the object. This also may not be desired.
A further known image stitching method for aligning a first sub image to a second sub image comprises the following steps: detecting a first type of output radiation, generating a first sub image using the first type of output radiation, detecting a second type of output radiation, generating a second sub image using the second type of output radiation and determining a shift between the first sub image and the second sub image. However, this known method requires extensive intervention of a user of a particle beam device used for carrying out the known method.
Another known image stitching method requires an extensive prior knowledge of the sample to be analyzed such that an optimum tilting of an imaged region of interest of the sample can be deduced, thereby avoiding having undesired features in the common region. Undesired features may lead to a false relative alignment of the sub images to each other.
As regards prior art, reference is made in particular to patent documents EP 2 660 845 A1, U.S. Pat. No. 8,767,038 B2 and U.S. Pat. No. 9,029,855 B2 as prior art.
In light of the aforesaid, it is desirable to provide a method for generating a composite image of an object and a particle beam device for carrying out the method which provide for an unique alignment of sub images to each other, even if no or unusable features are given in the sub images as orientation marks for the alignment, wherein the method is less extensive than the prior art.